The racemic compounds 3-substituted methyl-5-methylthio-2,3-dihydroimidazo1,2-c!quinazoline (I) (Chern, J.-W. et. al. J. Med. Chem. 1993, 36, 2196-2207) and 3-substituted methyl-2,3-dihydroimidazo1,2-c!quinazolin-5(6H)-one (II) have been recently synthesized as a conformational restricted analogs of 3-4-(2-methoxyphenyl)piperazin-1-yl!ethyl!quinazoline-2,4-dione, (SGB-1534, III) (Imagawa,J.; Sakai, K. Eur. J. Pharmacol. 1986, 131, 257-264) and have been shown to exhibit high potency and selectivity toward .alpha..sub.1 -adrenoreceptor. One of these compounds is currently undergoing intensive studies as an antihypertensive agent. The stereochemical requirements for biological activity remained to be clarified. ##STR2##